This invention relates generally to data signal detectors and, more particularly, to an improved method and apparatus for detecting the validity of a data signal transmitted on a noisy communication channel, which may also transmit speech on a time-shared basis. Such channels are found in some mobile and portable radio telephone communication systems.
Frequently mobile radio telephone systems, particularly cellular radio telephone systems, have the data in the form of Manchester encoded signals. With Manchester encoded signals, a 1 bit is indicated by a rising pulse edge at the center of a bit period and a 0 bit is indicated by a falling edge at the center.
Manchester encoded data is usually made up of data frames. Each data frame includes in series a bit stream of alternating ones and zeros called a "dotting pattern", a synchronization word, sometimes called a "Barker word", and finally a stream of data forming a data word. The total frame may last, for example, 100 microseconds. After the data frame passes, a speech signal may occur or the data frame may be repeated.
When receiving and decoding Manchester encoded data that is mixed in with voice signals on a time share basis, it is necessary to have a signal that indicates whether or not the received signal is valid data. A "data valid" signal can be generated by detecting the receipt of the dotting pattern or the Barker word. For example, a tone detector can be used to detect the dotting pattern. If the bit frequency of the data signal is 10 KHz, then a 5 KHz tone detector can be used to detect the 101010.. dotting pattern that starts data transmission, because the signal has edges only at the bit period centers and hence is really a 5 KHz signal. Such a solution to the problem data detection requires analog circuitry to implement it, which circuitry is generally difficult to design and construct. Also this technique is not accurate because voice signals or noise can sometimes have the same pattern as the dotting pattern.
A sync signal detector looks for a unique combination of bits. The received signal is decoded and compared to the predetermined code or bit combination. When this code combination is found, it is assumed that valid data has been located. Like dotting pattern detection, sync word detection can give false indications that the start of a data frame has occurred when the speech or noise randomly produce the required pattern for a sync word.
In prior art systems, the detection of a dotting pattern and sync word create a signal which indicates to the system processor that a data frame is being received. In response, the received signal is sent to the processor which then checks the data bit-by-bit using its internal program.
An integrator method can be used to process incoming Manchester signals to determine single bit validity. In such a circuit, a received signal is applied to one input of an Exclusive-OR circuit and a clock signal from a phase-locked loop is applied to the other input. If the signal contains valid Manchester data, the output of the Exclusive-OR circuit will remain high, if the received bit is a 1, and will remain low, if the receive bit is 0. The output signal of the Exclusive-OR circuit is then integrated. For example, this signal is used to enable a digital counter synchronized to the beginning of the bit period and clocked at a frequency F, which is many times, e.g. 40 times, the data bit rate. The counter should count to F when a 1 signal is received from the Exclusive-OR circuit and should count to 0 when a 0 signal is received, provided the data is perfectly received. When there is noise or shifting of the data edges, the count from the counter will be somewhere between 0 and F. If the count is below a preset value, the data is declared a valid 0 and if it is above a preset value, it is declared a valid 1.
In a prior art decoder where F=40, a limit for the count was set at 20, with a count greater than 20 indicating a 1 signal and a count less than 20 indicating a 0 signal. A major problem with such an integrator method is that the integrator counter circuit output indicates nothing about the appearance of the received wave form. Without tracking the location of the edges of the received signal, it is only possible to make a reasonable estimation as to the validity of the signal which the integrator has decoded.
In using the tone detector method for recognizing the dotting pattern and the integrator method for determining if a bit is valid, a period of time must elapse before the processor can determine if it is actually decoding a data frame. Depending on the limit set in the system, it is possible that the processor will not determine that the signal it is decoding is not a data frame soon enough to reset and then capture the actual data frame. In such a case the circuit may actually miss validly transmitted data.
Another problem exists in these systems when speech and data transmission share the same channel. It is desirable to separate the speech and data signals so that the data signal is not heard at the receiver. Thus, during the time period when a data frame is received, the processor mutes the audio in the headphone or speaker of the receiver. However, this cannot be done if false data detection occurs.
To overcome some of the problems of false detection, more sophisticated techniques are used. These techniques generally involve correlating the occurrence of a dotting pattern and/or Barker word to particular time periods and correlating the sequences that these signals occur in to that expected for valid data. U.S. Pat. No. 4,312,074 describes a method and apparatus for detecting a data signal, including repeated data words, which are mixed in with noise or a speech signal. The patent describes a program solution for microprocessors that uses a complicated algorithm to determine if valid data is being received. According to this system, upon the detection of each Barker word, the microprocessor stores the following data word and measures the elapsed time interval between data words by measuring the time between the detection of Barker words. The elapsed time interval is added to the stored time interval of all previously received data words. If at least 3 of the received data words have corresponding stored time intervals that are correlated with predetermined time interval ranges, a correlation indication signal is provided to indicate that valid data words have been received.
It can be seen that the prior art data valid detector circuits for Manchester encoded data either function poorly in the presence of noise or speech signals, or are extremely complicated. Thus, it would be beneficial to have a data valid detection circuit for Manchester encoded data that rapidly and accurately determines if valid data signals are being received, is simple in construction and operates accurately in the presence of noise.